“Cat’s pee on a gooseberry bush” sounds less than appealing, but it’s been quoted enough to become a cliché description for Sauvignon Blanc—even among people who’ve never tasted a gooseberry. I learned more personal grape-aroma associations from Master Sommelier Greg Harrington. He described red Bandol from Provence as smelling “like the burnt ends of a grandmother’s pot roast;” he also said Viognier often smells like Fruit Loops. These associations may seem far-fetched, but at the time, they helped Greg become the youngest person to ever pass the Master Sommelier exam, including the brutal blind-tasting portion.

Identifying a scent in a wine relies on the idea that a grape or a region’s wines have typical, identifying aromas or characteristics. In the MS exam, aspiring Master Sommeliers taste six wines in 25 minutes, hoping to identify each wine, right down to the vintage and sometimes even the vineyard, by analyzing its aromas, body, color, etc. A difficult task, entailing a large dose of Sherlock Holmes-like deduction.

Aromatic descriptors don’t need to be off-the-wall to be typical to their grape variety or region. Generally, the wine industry today favors relatively objective tasting notes over more poetic, idiosyncratic descriptions—that pot roast would probably be transformed into “rich, meaty aromas” in print (or even during an MS Exam). Cabernet Sauvignon often has an aroma of blackcurrants; Brunello di Montalcino, a touch of graphite; New Zealand Sauvignon Blanc can have aromas of passionfruit and grapefruit, or, in cooler vintages, the smell of green vegetables like peas, asparagus, or even grass—which is better than cat pee, at least.

Things can get even more specific. Burgundy, for example, is famous for characterizing its vineyards and their character. The wines of the village Volnay are noted for their cherry aroma, whereas Gevrey-Chambertin reveals itself with a touch of licorice. Then, within the latter village, the Latricières-Chambertin vineyard is held to yield a spicier wine, whereas nearby Clos de Bèze is more floral. Different grape varieties? No, they’re all Pinot Noir.

Of course, a single wine will have more than one aroma: Gewürztraminer is known for its giveaway scent of lychee, but tangerine, rose petal, or mandarin notes also fit the profile. An MS examinee may i.d. every scent their nose can pick up, but just a few well-chosen descriptors can be adequate to distinguish between different wines for a restaurant guest: “This Cab is round and rich, with lots of dark fruit aromas, whereas the other is spicier and more tannic.” Job done.

Non-professionals often say, “I don’t see how people get all these different aromas in wine; I don’t smell all that.” It’s not the sommelier or server’s job to teach the guest, but they have a point. After all, raspberry wine tastes like raspberries and blueberry wine, like blueberries. But wine made from grapes rarely actually tastes grapey.

In 1990 UC Davis professor Ann C. Noble introduced the Wine Aroma Wheel, which subdivides potential wine aromas into broad categories (fruity, floral, earthy, etc.) and then breaks down each one into sub-categories and individual descriptors, like raspberry, rose petal, etc. It’s especially useful for people who are still developing their wine vocabulary; often times we pass over aromas not because we don’t smell them, but because the words don’t come to mind readily enough. Some of the descriptors on Noble’s Wheel are faults (aromas only found in an improperly made or tainted wine) but she included 87 potential aromas in total (note: only one of them is “grape”). That’s a conservative number, and most wine pros parse things a bit further. Apple isn’t specific enough: is it Granny Smith? Baked? And the wheel doesn’t even include cat pee, though wet dog does get a mention.

Where are all these aromas coming from? In theory, it’s simple: aromas are chemicals. What’s surprising is how hard it can be to determine what chemical is behind a specific aroma in a wine. Researchers have discovered a few. The touch of green bell pepper that shows up in unripe Cabernet? Methoxypyrazines. Pineapple aromas in a Chardonnay? Ethyl Caprylate. (No word yet on the mysterious “grandmother’s pot roast” molecule).

But other factors can influence how or whether we perceive these aromas, so it’s not as simple as ticking off a list of chemicals present. A small change in temperature can change our perception; that’s one reason wines often taste different than they smell—the wine warms up in our mouth. A lot of chemical interactions also take place, during winemaking as well as during aging. For example, varying combinations of acids, which may or may not have aromas of their own, also stimulate the formation of different esters, a group of chemicals responsible for many of wine’s fruity aromas (over 160 esters have been found in different wines).

These aromas only appear after the grapes become wine because many of the relevant chemicals are bound to sugar molecules inside the grape, which renders them odorless until released by fermentation. This is one of the big differences between grape wine and wine made from other fruits. The other big factor is the winegrape’s balance of sugar, acidity (especially tartaric acid, which is rare in other fruits and particularly resistant to bacteria), and yeast nutrients; grapes, more than other fruits, lend themselves to making a stable wine.

As for varietal diversity, it’s not really that surprising. It seems the varieties used today were selected from wild vines a long time ago; we’ve been making wine for at least six millennia, so they’ve had quite a while to grow apart and distinct. If we can breed dogs into St. Bernards, Chihuahuas, and Greyhounds, it’s not all that amazing that we got some variety out of something that doesn’t run away when it’s off the leash.